1. Physics 4 – Jan 31, 2017 P3 Challenge –
A R1 = 4 , R2 = 3 , and R3 = 6  are connected to a 18V power source as shown. Solve the resulting circuit.
Get out Circuit WS #14-24 for HWK check

2. Objectives/Agenda/Assignment
5.3 Batteries
Assignment: p230 #30-36
Agenda:
Homework Review
EMF
Ideal Batteries
Real Batteries
Capacity and charging

3. Electromotive Force
An EMF doesn’t have to be a battery, it’s just that batteries are by far the most common.
What is needed is a source of a potential difference, an electromotive force (EMF), symbolized by .
An EMF converts some other form of energy into electrical energy.
EMF source
Type of energy converted
Battery
Chemical
Generator
Mechanical
Thermocouple
Thermal energy
Solar cell / photovoltaic cell
Light energy

4. Ideal Batteries I
The chemical reaction occurring within a battery is an oxidation reduction reaction that serves to move ions from one location to another.
The positive terminal (the cathode) is where a reduction occurs. (Electrons move onto chemicals.)
The negative terminal (the anode) is where an oxidation occurs. (Electrons are removed from chemicals.)
The specific chemicals used in the redox reaction identifies the names used for batteries: Cd, Zn, Li etc… - +

5. Types of Batteries

6. Real batteries V =  - Ir V =  only if the current = 0
Real batteries have an internal resistance.
The terminal potential difference, V, is not exactly equal to the electromotive force, , but is something less because some energy gets dissipated by the internal resistance, r.
V =  - Ir
V =  only if the current = 0
Treat the internal resistance as another resistor connected in series next to the battery.

7. Sample Problems
A R1 = 4 , R2 = 3 , and R3 = 6  are connected to a 18V power source as shown. Solve the resulting circuit if the battery has an internal resistance of .
Find the EMF of the battery and the internal
resistance for the graph shown on the last slide.
When two resistors, each of resistance 8  are connected in parallel with a battery, the current leaving the battery is 2.8 A. When the same two resistors are connected in series with the battery, the total current in the circuit is 0.65 A. What is the EMF of the battery and the internal resistance?

8. Capacity Batteries come in two varieties: Primary and secondary.
Primary cells/batteries have a fixed amount of charge they can deliver because their chemical reaction can only go forward. Once the chemicals have reacted, the battery is spent and can only be disposed.
Secondary cells/batteries are based on reversible chemical reactions that can be run backwards. These are the rechargeable batteries.
The terminal potential varies some as a cell discharges.

9. Charging a battery
You can charge a secondary battery by placing it “backwards” in the circuit.
Remember the sign conventions for Kirchhoff’s loop rule?
A battery that is backwards drains potential.
That energy loss is transferred into chemical energy recharging the battery.

10. Exit slip and homework
Exit Slip – A 30  resistor and a 50  resistor are connected to a 12 V battery with a 1  internal resistance. Solve this circuit.
What’s due? (homework for a homework check next class)
Ch 5.3 p230 #30-36
What’s next? (What to read to prepare for the next class)
Read Ch 10.1 p